1. Technical Field
The present embodiment generally relates to a method of initiating channel decode in demodulators of a television system and, more particularly, relates to a method to invoke channel decoder early to decrease acquisition time in demodulators using synchronisation boundaries.
2. Description of the Related Art
Digital Video Broadcasting-Terrestrial/Handheld (DVB-T/H) digital demodulation system, a channel decoder includes Viterbi Decoding, followed by Reed Solomon decoder (RS-Decoder) and De-randomisation. To initiate Viterbi decode and RS-Decode initial state setups are required. Start of a channel super frame is the synchronisation boundary for the channel decoder. The initial conditions for Viterbi decode, RS and de-randomization is satisfied at this boundary. Super frame constitutes 4 frames. In DVB-T 8K mode, ¼ Guard Interval, super-frame boundary interval is 304 ms while it 76 ms for 2K mode, ¼ Guard interval. A search for this synchronisation boundary adds to overall channel change times.
Currently, channel decoders are synchronised based on super-frame boundary of an OFDM signal data. FIG. 1A illustrates a generic DTV system. It includes a RF tuner 102, a Demodulator 104, a media processor 106 and a display 108 with speakers 110. The RF tuner 102 tunes to a programmed RF frequency and outputs required frequency spectrum at a lower or standard Intermediate Frequency (IF) signal. The demodulator 104 receives the low IF or the standard IF data/signal and demodulates the IF signal and outputs a Transport Stream (TS). The Media processor 106 decodes the TS data and outputs video and audio to the display 108 with the speakers 110.
With reference to FIG. 1A, FIG. 1B is a flowchart illustrating a method of synchronising at a De-randomiser. In step 112, an RS decoder output 110 is subjected to detect SYNC′. If SYNC′ is not detected, the RS decoder output sends a fresh output data for detecting SYNC′ until SYNC′ byte is obtained. After the SYNC′ is detected by the RS decoder output in step 110, a pseudo-random binary sequence (PRBS) is initialised in step 114. The PRBS output initialises the Derandomiser in step 116.
FIG. 2A illustrates a block diagram of a typical DVB-T/H demodulator system. The block diagram includes a tuner 202, a signal conditioning and baseband conversion block 204, a mode and GI detection block 206, a Fast Fourier Transform (FFT), pilot and TPS processing block 208, a frequency/sampling time locking block 210, a channel estimation block 212, a demapper/symbol and deinterleaver block 214, a Viterbi decoder block 216, an outer deinterleaver block 218, and a RS decoder and derandomizer block 220. Any channel change (tuning to different frequency spectrum) requires settling timings for the tuner 102, the demodulator 104, and the media processor 106. These all timings will add up to the new channel video/audio rendering time which may be irritating for a viewer. In any receiver systems, before performing decoding/demodulation, systems need to be synchronised to a known point of a transmitted signal.
FIG. 2B illustrates a plurality of Scattered Pilots (SP) used for Even/Odd Symbol Detection. Reference information, taken from a reference sequence, is transmitted in Scattered Pilot cells in every symbol. Scattered Pilot cells are always transmitted at a “boosted” power level. Thus the corresponding modulation is given by:Re{cm,l,k}=(4/3)*2((½)−wk)Im{cm,l,k}=0Where m is the main frame index, k is the frequency index of the carriers and 1 is the time index of the symbols. For a symbol of index 1 (ranging from 0 to 67), carriers for which index k belongs to the subset {k−Kmin+3*(1 mod 4)+12p, p is an integer, p>0, k[Kmin: Kmax]} are scattered pilots. P is an integer that takes all possible values greater than or equal to zero, provided that the resulting value for k does not exceed a valid range [Kmin:Kmax].
The purpose of the symbol interleaver 214 is to map ‘v’ bit words onto the 1512 (2K mode) or 6048 (8K mode) active carriers per OFDM symbol. The symbol interleaver 214 acts on blocks of 1512 (2K mode) or 6048 (8K mode) data symbols. Thus, in the 2K mode, 12 groups of 126 data words from the bit interleaver 214 are read sequentially into a vector Y′=(y′0, y′1, y′2, . . . y′1 511). Similarly in the 8K mode, a vector Y′=(y′0, y′1, y′2, . . . y′6 047) is assembled from 48 groups of 126 data words.
The interleaved vector Y=(y0, y1, y2, . . . yNmax−1) is defined by:yH(q)−y′q for even symbols for q=0, . . . , Nmax−1yq=y′H(q) for odd symbols for q=0, . . . , Nmax−1Where Nmax=1 512 in the 2K mode and Nmax=6 048 in the 8K mode.
FIG. 2C illustrates a plurality of Reed Solomon Packets with SYNC/SYNC′. It includes four different packets viz: (i) a MPEG-2 transport MUX packet 222, (ii) Randomised transport packets 224, (iii) Reed Solomon RS (204, 188, 8) error protected packets 226 and (iv) a data structure after outer interleaving 228. The Randomised transport packets include SYNC bytes and randomised data bytes. The interleaving depth (I) in the data structure after outer interleaving 228 is 12 bytes. FIG. 2D is a table view illustrating a puncturing pattern and transmitted sequence after parallel-to-serial conversion for a plurality of possible code rates. The table view includes a code rate R field 230, a puncturing pattern field 232, a transmitted sequence field 234
FIG. 2E illustrates convolutional code of rate ½. Upon obtaining the transmitted sequence 234 after parallel-to-serial conversion as X1Y1, X1 is received first by Viterbi Decoder input which starts at SYNC/SYNC′. SYNC implies TS sync byte 0x47 and SYNC′ implies bit inverted TS sync byte 0XB8. At start of a super frame, MSB of SYNC/SYNC′ lies at data input 236. The data input goes through series of 1-Bit delay processing with Modulo-2 addition 238 to produce X output (G1=171 Octal) and Y output (G2=133 Octal). The first convolutionally encoded bit of a symbol always corresponds to X1 by default.
Thus, a channel decoder starts at a channel super frame level which takes time in searching and decoding and thereby increases channel scan/change timings. Accordingly, there remains a need to develop systems and method of efficient and timely invoking of channel decoder in order to reduce channel change/scan times.